The present invention relates to a variable displacement compressor used in a vehicle air conditioning system, and more specifically, to a variable displacement compressor that has a displacement control valve for controlling the displacement of the compressor.
A variable displacement compressor used in a vehicle air conditioning system is driven by a vehicle engine. The displacement, or cooling performance, of the variable displacement compressor is automatically controlled based on cooling load. A swash plate type variable displacement compressor has a swash plate located in a crank chamber. The inclination of the swash plate is altered by controlling the pressure in the crank chamber with a specially designed control valve. Altering the swash plate inclination changes the stroke of pistons, which varies the displacement of the compressor. The specially designed control valve can be an internally controlled valve or an externally controlled valve.
An internally controlled control valve includes a pressure sensing mechanism. The pressure sensing mechanism sets a target pressure and detects the gas pressure in a suction chamber of the compressor, or the suction pressure. The pressure sensing mechanism is displaced by the difference between the target pressure and the suction pressure, which automatically changes the opening amount of the control valve. The target pressure of the internally controlled control valve cannot be changed externally. It is sometimes desirable to change the displacement of a compressor in accordance with the running state of the engine regardless of the suction pressure, which represents the cooling load. However, if the compressor has an internally controlled control valve, the compressor displacement cannot be controlled based on the engine running state since the target pressure cannot be changed externally.
An externally controlled control valve includes a pressure sensing mechanism and an electromagnetic actuator coupled to the pressure sensing mechanism. The displacement of the compressor is determined by a controller based on the running state of the engine and the running state of the vehicle. The controller then electrically actuates the electromagnetic actuator, accordingly. In this manner, the target pressure of the externally controlled control valve is determined in accordance with external factors. Thus, the displacement of the compressor is optimized for the running state of the engine. Specifically, when the vehicle requires a relatively great amount of power, for example, when the vehicle is rapidly accelerated, the load of the compressor on the engine can be reduced.
The pressure sensing mechanism includes a pressure sensing member, which is a bellows, and a spring located in the bellows. The bellows is displaced along its axis, or expanded and contracted, in accordance with the suction pressure. The electromagnetic actuator includes a solenoid and associated parts. The solenoid is axially aligned with the bellows.
The pressure sensing mechanism must be axially aligned with the electromagnetic actuator such that the bellows is axially aligned with the solenoid. This complicates the structure of the control valve and increases the number of parts. Thus, the cost and the number of assembly steps are increased. Also, the size of the compressor is enlarged. The controller has an amplifier to actuate the solenoid. Since the pressure sensing mechanism is actuated by the electromagnetic actuator, a relatively great electrical load is applied to the amplifier.